The present invention relates to an image forming apparatus such as an electrophotographic copier, laser printer or similar non-impact printer, or facsimile apparatus. More particularly, the present invention relates to an image forming apparatus of the type including an image forming case or drawer removably mounted on the body of the apparatus and in which a photoconductive element, developing unit and cleaning unit are arranged integrally and replaceably, a transfer charger unit and a register roller unit removably mounted on the apparatus body and accurately positioned relative to the photoconductive element, and a guide arrangement for guiding a paper sheet which is transported via the register roller unit to between the photoconductive element and the transfer charger unit while maintaining the paper in a predetermined position relative to the photoconductive element, transfer charger unit and register roller unit.
An electrophotographic copier, printer, facsimile apparatus and other electrophotographic image forming apparatuses are implemented by substantially identical image forming units, i.e. photoconductive element unit, developing unit, cleaning unit, charger unit, etc. These units are individually replaced from time to time for replacement or repair. A current trend in the art of image forming apparatuses, especially printers and electrophotographic copiers, is toward user-oriented maintenance. In parallel with such a trend, that each of the image forming units be replaceable with ease by a user is becoming a prerequisite. To meet this requirement, there has been proposed a process kit in which a developing unit, cleaning unit and other image forming units which need periodic replacement are assembled integrally with each other. The process kit is bodily replaced when any of the image forming units is to be replaced upon the lapse of a certain period of time. Such a process kit scheme, however, increases the parts cost and therefore the running cost considering the fact that the service life and fatigue of each image forming unit depends upon its characteristics and the condition of use. Specifically, it may occur that when a certain image forming unit reaches the end of its life and needs replacement, the others still withstand further operations. Replacing the entire process kit in conformity to an image forming unit which should be replaced first, i.e., discarding image forming units which are still usable increases the parts cost and therefore the running cost, casting an excessive burden on a user.
To eliminate such a drawback particular to a process kit, there has also been proposed a system in which a cleaning unit, developing unit, photoconductive element and other image forming units are each provided with a different replacement cycle and replaced independently of each other. For the replacement, two different approaches are available: mounting the image forming units on the apparatus body such that they may be pulled out independently of each other at the time of replacement, and mounting the image forming units on a single image forming case and removably mounting the case on the apparatus body so that any of the units may be replaced by drawing out the entire case.
The first-mentioned approach suffers from a drawback that each of the image forming units has to be equipped with an exclusive mechanism for pulling it out. Moreover, the image forming units have to be positioned with accuracy relative to each other and, since they are packed in a limited space, replacing the individual image forming units is troublesome. In addition, when a certain image forming unit is removed for replacement, it is apt to scratch or otherwise damage the others and/or toner is apt to drop to contaminate the interior of the apparatus. The second-mentioned approach is successful in positioning the individual image forming units with accuracy. Nevertheless, the replacement of each image forming unit with the second-mentioned approach is not easy and liable to cause dropping of toner and damage to the other image forming units.
It has been customary to mount the transfer charger unit and register roller unit on the apparatus body to facilitate the removal of a jamming sheet. Difficulty has therefore been experienced in maintaining an accurate positional relationship between the various image forming units mounted on the image forming case and the transfer charger and register roller unit, especially between the photoconductive element and a charge wire of the transfer charger unit. At least, an image forming apparatus having a body on which the transfer charger unit and register roller unit are removably mounted in addition to the image forming case has not been proposed. When, among others, the transfer charger unit is removably mounted on the apparatus body, it is extremely difficult to surely ground the casing of the unit.
An image forming apparatus of the type described includes a guide arrangement for guiding a paper sheet being transported via a register roller unit to between a photoconductive element and a transfer charger. The guide arrangement is usually implemented by an upper and a lower guide plate. These guide plates have to be adequately positioned relative to the photoconductive element and register roller unit. Otherwise, accurate transfer of a toner image would fail due to deviation or local omission of an image which is ascribable to the deflection of the paper sheet or the reaction force of the trailing edge of the paper sheet. To insure accurate transfer, therefore, it is necessary to constantly maintain the above-stated positional relationship accurate. However, a practical solution has not been reported concerning the most desirable positional relationship.